Abstract

The Northwest Geysers EGS Demonstration Project started in 2011 at The Geysers (California). The goal was to create an EGS by injecting cool water at relatively low pressure into a known high temperature reservoir located deep down below the conventional geothermal steam reservoir. Here, we present (1) tomography images of the seismic velocity (Vp, Vs) and attenuation (Qp, Qs) built from microseismic data recorded before injection and during the first 60 days of injection, (2) the spatiotemporal distribution of the microseismicity over 270 days of injection, and (3) a 3D thermo-hydro-mechanical modeling of the EGS. The results show the development of a seismically quiet domain around the injection well surrounded by a seismically active domain. The modeling with comparison to field data indicates that the seismically quite (aseismic) domain is due to the presence of the injected cold water. Thermal processes cause a cooling-stress reduction preventing shear reactivation. This process is accompanied by aseismic plastic shear strain, which can be interpreted as slow slip events. In the seismic domain, microseismicity is caused by the reactivation of the preexisting fractures due to injection-induced pressure increases in steam pressure.

1. INTRODUCTION

Injecting fluid into a rock formation reduces the effective stress and can lead to brittle failure [1]. Such fracture reactivation can be accompanied by microseismic events due to the release of preexisting tectonic stresses. The monitoring of microseismic activity can provide valuable information concerning the response of the reservoir to water injection. The occurrence of microseismicity at a point remote from the injection interval indicates that the point has a hydraulic connection to the borehole interval. However, it does not necessarily mean that significant fluid flow occurs along the connection [2]. Moreover, during the reservoir stimulation, aseismic deformations can play an important role on the reservoir development during hydraulic stimulation [3].

In this paper, we present data collected during the Northwest Geysers EGS Demonstration project [4,5]. The project aims to develop an EGS by directly and systematically injecting cool water at relatively low pressure into a vapor-dominated geothermal reservoir with temperatures up to 400 °C. We focus our study on the relationship between the formation of the liquid saturated zone and the distribution of the seismic and aseismic deformations. First, we present the Northwest Geysers EGS Demonstration project, a 3-D seismic tomography performed during the first months of injection and the microseismic activity by using the Seismicity-Based Reservoir Characterization approach [6, 7]. Finally, we discuss our observations with the results of a 3-D Thermo-Hydro-Mechanical (THM) numerical model simulation of the EGS area.

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